Mesh composite graft

ABSTRACT

A mesh composite graft including an inner component, an outer component formed from strands of durable material, such as polyethylene terephthalate, and an intermediate component made from strands of biocompatible synthetic material having a melting point less than that of the durable material from which the outer component is formed and less than that of the biocompatible synthetic material from which the inner component of the graft is formed. By heating the graft to a temperature greater than the melting point of the material from which the intermediate component is formed but less than the melting point of the outer component material and less than the melting point of the material from which the inner component is formed, the components are bound by the melted intermediate component to provide a totally porous, compliant composite graft reinforced by the outer component.

This application is a divisional of copending application Ser. No.634,425, filed Dec. 27, 1990, now U.S. Pat. No. 5,116,360.

BACKGROUND AND DESCRIPTION OF THE INVENTION

The present invention generally relates to implantable prostheses andthe like and to methods for making same. More particularly, theinvention relates to a graft, such as a vascular graft or AV-shunt,having a compliant porous inner component and a compliant porousload-bearing outer component, bound together by a porous intermediatecomponent that is made of material having a melting point lower thanthat of the materials from which the inner and outer components aremade. With the outer component bound by the intermediate component tothe inner component, a porous, yet strengthened integral graft results.

Blood vessels are not straight, rigid tubes but elastic conduits made ofa variety of materials and having a compliance that varies withfunctional considerations. For example, the venous system functions, inpart, as the blood reservoir for the body. In order to be able torespond to a larger volume of blood sent into the system because of, forexample, a change in arterial blood pressure, the vessels of the venoussystem must be sufficiently compliant so that they can distend. Thearterial system functions as the body's pressure reservoir. In order toavoid the wide swings in the blood pressure and flow that are possiblewith every contraction and relaxation of the heart, yet be able tomaintain sufficient blood pressure so that blood can be pushed into allregions of the body, including through the small-diameter arterioles andthe microcirculatory bed, the arteries must have sufficient compliantstrength to elastically expand and recoil without the marked distensionof the venous system.

Conventional grafts, however, are generally made of materials and inshapes that provide a structure whose compliance is markedly differentfrom that of the walls of the vessel to which they may be attached.Grafts having walls less compliant than that of the host vessel wallsare problematic in that conditions, such as intimal hyperplasia andstenotic narrowing, may develop. Grafts with walls having greatercompliance than that of the vessel to which the graft is attached areproblematic in that a portion of the graft wall may balloon--that is,develop an aneurysm--after implantation.

Other known grafts, while they may be compliant, may not necessarily bemade from biocompatible materials. The implantation of a graft made fromsuch material may prompt a thrombogenic or immunological response withthe resultant deleterious formation of microthrombi or microocclusionsin and around the graft. Other grafts are made from generally non-porousmaterials, that, accordingly, do not facilitate the ingrowth of cellsand tissue within the graft. The full incorporation of the graft intothe surrounding host tissue is thereby frustrated. Still otherconventional grafts are made from microporous textiles that requirepreclotting of the vessel wall with blood to prevent leakage of blood atimplantation.

A demand therefore is present for an integral graft made frombiocompatible materials and having a structure that has compliantstrength similar to that of natural tissue but that is sufficientlyporous so that the graft may become incorporated into the host tissueyet not leak blood. The present invention satisfies the demand.

The present invention includes a three component system, an innercomponent, an intermediate component, and an outer component. While thecomponents may be made from materials having generally different meltingpoints and different mechanical properties, at a minimum the innercomponent and outer component are made from a material or materialshaving a melting temperature higher than the material from which theintermediate component is made. More specifically, the inner componentis porous and is made from a biocompatible synthetic material,preferably a polyurethane composition made with an aromaticpolycarbonate intermediate, having a melting point that is, at aminimum, in excess of the melting point of the composition from whichthe intermediate component is formed (further discussed below).

There are many methods by which the inner component may be made, such asthe many known methods used to produce porous compliant vascularprostheses. One such method is termed phase inversion or separationwhich involves dissolving a urethane in a solvent, such as dimethylacetamide (DMA), forming a coat on a mandrel--such as by dipping themandrel into the dissolved urethane--and then immersing the urethanecoating in a solution such as water by which DMA may be dissolved, butnot urethane, thereby causing the urethane to bead-up and form a porousmatrix.

Another method by which the inner component may be formed is termedparticle elution. The method utilizes water soluble particles such assalt (NaC1, MgC12, CaCo2, etc.) polymers, such as polyvinylpyrrolidone,sugars etc. The particles are mixed or blended into a urethanecomposition, and after forming a graft from the mixture such as by dipcoating or extruding the particle filled plastic, the particle is elutedout with a suitable solvent.

Additional methods include replamineform, that involves the dissolutionof a matrix, such as that of a sea urchin, out of the urethane withhydrochloric acid, spray techniques where filaments or beads of urethaneare sprayed onto a mandrel to produce a porous vascular graft, andelectrostatic deposition of urethane fibers from solution.

However, the porous vascular graft preferred in this invention isprepared according to the method detailed in U.S. Pat. No. 4,475,972 toWong. This patent is incorporated hereinto by reference. An antioxidantmay be added to further prevent degradation of the fibers drawn of thematerial from which the inner component is made.

Regardless of the nature and method of manufacturing the porous innercomponent, the intermediate component is comprised of one or more layersof a biocompatible synthetic material, preferably a polyurethanematerial, having a melting point lower than the melting point of thematerial from which the inner component is formed and lower than themelting point of the material from which the outer component is made.

The outer component comprises a mesh network made of strands, fibers,beads or expanded versions of a durable material such as a compositionof fluorocarbons, such as expanded polytetrafluoroethylene("ePTFE")--commonly termed Teflon--or stable polyesters, such aspreferably polyethylene terephthalate ("PET")--commonly termed Dacron.This material is preferably warp-knitted in a tricot or double tricotpattern and shaped in a tubular configuration. It can also beappreciated that the outer component can be woven, braided, weft-knittedand the like with loose fibers, textured fibers and the like to provideincreased compliance. With the three components in place, a compositegraft according to the present invention is formed by heating thestructure to a temperature at or above the melting point of the materialfrom which the intermediate component is formed but below the meltingtemperature or temperatures of the material from which the outercomponent is formed and of the material from which the inner componentis formed. In this temperature range, the intermediate component maymelt without the melting of either the inner component and the outercomponent, thereby mechanically bonding the inner component to the outercomponent.

The multi-component system of the present invention provides a number ofadvantages over conventional grafts. The use of a durable material, suchas PET or ePTFE, from which the outer component may be formed isadvantageous because of the known strength that such material has in thebody. Devices made from PET or ePTFE when implanted in the body areknown to maintain their integrity for some three decades. Furtheradvantageously, it has been found that a graft--made according to thepresent invention and in which PET is used to form the outer component--has a burst strength and a tensile strength that is some two timesgreater than that of a conventional graft. Such strength prevents thedilation of the vessel in response to, for example, an increase in bloodflow and/or pressure, creep relaxation of the urethane, biodegradationof the urethane, plasticization of the urethane, etc. Decreases in thestrength of PET that may occur after implantation due, for example, tothe absorption of water after implantation, are minimal as Dacron has alow water absorption ability.

The use of a knitted pattern according to which the durable strands ofthe outer component may be configured is advantageous due to theincreased compliance such a pattern provides. As stated above, a durablematerial such as PET is recognized as a strong yet not necessarilycompliant material. However, by knitting the strands from which theouter component is formed into a network, a compliant reinforcing outercomponent is formed. The use of such a material from which to form theouter component in the three component system of the present inventionadvantageously provides a strengthened, yet compliant graft.

The winding of strands of synthetic material, such as polyurethane overa mandrel to form an inner component is further advantageous because ofthe resultant porosity of the component. While the intermediatecomponent may be made porous, for example, by painting syntheticmaterial over the inner component and utilizing the phase inversionmethod or the particle elution method to form a porous matrix,preferably the intermediate component is formed by winding strands ofsynthetic material, such as polyurethane over the inner component, toprovide a highly porous network. Utilizing strands of PET configured ina knitted pattern to form the outer reinforcement component furtherprovides a porous network. Advantageously, by combining theseindividually porous components together in a composite graft, a totallyporous integral graft results. Porosity is an advantage in medicaldevices, such as vascular grafts, because an open structure allowsvascular fluid to infiltrate and communicate to and from the surroundingtissue and the interior of the graft and allows the ingrowth of tissueto occur within the graft. Accordingly, the device becomes betterincorporated into the surrounding tissue, thereby further securing thedevice within the implantation site.

Uniting the three components into a single composite graftadvantageously facilitates the use of the device. The graft may beimplanted without the need for any assembly immediately prior to use.The graft may be also cut and/or sutured as a unit without the need forthe separate cutting and/or suturing of each component. Methods forcutting the composite graft include scalpel, scissors, hot wires, shapedblades, and the like. The speed with which the graft may be implanted isa particularly distinct advantage since the device is implanted onlywhen a patient is undergoing surgery.

The use of a polycarbonate intermediate rather than, for example, apolyether urethane to make the polyurethane material from which theinner component is preferably made is advantageous as the resultantinner component better resists degradation. The resistance todegradation is further aided by the addition of antioxidant to thematerial from which the inner component is formed.

It is, accordingly, a general object of the present invention to providean improved graft.

Another object of the present invention is to provide an integralimproved graft made from a composite of layers of synthetic materials.

It is also an object of the present invention to provide a graft that istotally porous thereby facilitating the incorporation of the graft intothe site of implantation.

An additional object of the present invention is to provide an improvedgraft having an outer component which strengthens the device withoutsignificantly impairing the overall compliance of the graft.

These and other objects, features and advantages of this invention willbe clearly understood and explained with reference to the accompanyingdrawings and through a consideration of the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of this description, reference will be made to theattached drawings, wherein:

FIG. 1 is a perspective view illustrating an embodiment of a compositevascular graft according to the present invention with an outercomponent of knitted durable material positioned over and bound by anintermediate component to an inner component; and

FIG. 2 is a cross sectional view of the composite vascular graftaccording to the present invention illustrated in FIG. 1.

DESCRIPTION OF THE PARTICULAR EMBODIMENTS

The present invention is a composite vascular graft--generallydesignated as 21 in FIGS. 1 and 2--comprised of an inner component 31,an intermediate component 41, and an outer component 61. The innercomponent will be described first.

Inner component 31 is fabricated from a biocompatible syntheticmaterial, preferably polyurethane, having a melting temperature that is,at a minimum, greater than the melting temperature of the material fromwhich the intermediate component is formed. Preferably, in thoseembodiments in which the inner component 31 is formed from polyurethane,it is made with an aromatic polycarbonate urethane. Polycarbonateurethanes are preferred over polyether urethanes due to their superiorbiostability. The aromatic polycarbonate urethanes have melting pointsin the range of 150° C. to 230° C. This is in contrast to some aliphaticpolycarbonate urethanes that have melting points between 90° C. and 130°C. It can also be appreciated that the inner member may be composed ofnon-urethane materials such as silicone rubber, polyolefins,fluoroelastomers, ePTFE, and the like. An antioxidant, such as Irganox1010, may be added to the inner member to further prevent degradation ofthe strands from which the inner component is formed. The meltingtemperature of the material from which the inner component is preferablyformed exceeds 150° C.

The methods by which the inner component 31 may be fabricated includethose disclosed in U.S. Pat. No. 4,475,972 to Wong. According to afabrication method taught in the Wong patent, termed "solutionprocessing", the inner component material is dissolved in a solvent andforced out of one or more orifices to form one or more continuousfibers. The fibers are drawn directly onto a rotating mandrel. As thedistributor or spinnerette reciprocates along the mandrel, non-wovenstrands are layered on top of each other to form porous, non-wovennetwork of strands.

The intermediate layer 41 is formed of a biocompatible syntheticmaterial, such as a polyolefin, a silicone thermoplastic material, etc.,or preferably a polyurethane material having a melting temperature lessthan that of the materials from which the inner and outer components areformed. The intermediate layer can be drawn in the manner described inthe Wong patent so that at least one fibrous layer is laid over theinner component 31 to form a porous intermediate layer. Thisintermediate layer can be spun from solution as described in the Wongpatent or can be simply wound onto the inner layer from a spool of thebiocompatible low melting point material. Alternatively, phase inversionor particle elution methods may be used to form a porous intermediatecomponent. Examples of suitable low melting point biocompatiblematerials include the aliphatic polycarbonate or polyether urethaneswith melting points of 90° C. to 130° C. The resultant porous, non-wovennetwork of strands forming the intermediate component 41, as drawn overthe inner component 31 form a unit 51 which facilitates the transmissionof fluid.

Mesh 61, composed of strands of durable material, such as PET or ePFTE ,knitted or woven in a generally elongated cylindrical shape and whoseinner surface 63 is of a diameter equal to or slightly larger than thediameter of the outer surface 45 of the intermediate component 41, isfitted over the intermediate component 41. To provide compliance to themesh network of strands from which the outer component is formed, thestrands are configured preferably in a knitted pattern. Tricot or doubletricot warp knit patterns are preferred. Double tricot patterns arefurther advantageous because they provide greater depth to the outercomponent 61 and thereby facilitate the acceptance of and retention ofsutures and tissue ingrowth through the graft 21. Tricot or doubletricot warp patterns are further advantageous in that they are generallymore interlocking than other patterns and therefore resist "running".Other acceptable patterns according to which the strands of the outercomponent 61 may be formed include jersey or double jersey patterns,woven or braided and multiple layers of the above. Also, the fiberscomprising the outer structure may be textured or non-textured and be ofa variety of deniers.

The outer component 61 as positioned over the inner component andintermediate component is heated to a temperature equal to or greaterthan the temperature at which the material from which the intermediatecomponent 41 is formed melts but less than the temperature and/ortemperatures at which the material or materials from which the outercomponent and from which the inner component 31 is formed melts. Whenthe inner component 31 is formed from the preferred material describedabove, the components are heated to a temperature less than 150° C. butgreater than the temperature at which the material from which theintermediate component 41 is formed melts, such as 110° C. Bymaintaining the three components at such a temperature for a period oftime, such as ten minutes, the intermediate component melts therebysecuring the outer component 61 and the inner component 31 to eachother. To further ensure the secure full engagement of the outercomponent 61 by the melted intermediate component 41, the outercomponent 61 may be forcefully pressed into the intermediate component41 during the heating step such as mechanically and/or with or underpressure. After heating, the united three components are cooled therebyproviding an integral mesh composite graft 21.

A mesh composite graft 21 according to the present invention is totallyporous and compliant, yet advantageously includes a load bearingcomponent, the outer component 61, which adds strength to the graft andprevents the failure of the graft even in response to greater fluidvolume pressures from within, creep relaxation of the inner member andpossible biodegradation effects of the inner member.

The advantageous compliance of the composite graft may be adjusted byvarying the number of strands from which the inner component and theintermediate component 41 are formed. The compliance of the compositegraft 21 may be adjusted also by varying the materials from which theinner component 31 and the intermediate component 41 are formed whilemaintaining the relationship that the intermediate component 41 mustmelt at a lower temperature than the materials from which the outercomponent and the material from which inner component 31 is formed. Thecompliance of the mesh composite graft 21 may be adjusted further byadjusting the angle at which the strands of the inner component 31and/or the strands of the outer component 61 are laid down--a higherangle provides a less compliant component and thereby a less compliantgraft.

The compliance may be adjusted even further by altering the knittingparameters, such as courses and wales per inch, the stitch density, thefiber denier, the number of strands per filament, the composition of thefibers and filaments such as a mixture of PET and Spandex compositionsand whether the outer member is knitted, woven or braided.

The advantageous overall porosity of the graft 21 may be adjusted alsoin a number of ways. In addition to varying the size and number of thestrands from which the inner component 31 and intermediate component 41are formed, the strands of each component may be drawn at differentangles to provide decreased pore size and resultant decreased porosity.Similarly, the porosity of the outer component 61, and thereby theporosity of the composite graft 21 may be varied by varying the sizeand/or number of the strands and stitch density used to make the outercomponent mesh.

It can also be appreciated that the outer component need not be a tubeformed specifically for this purpose from materials as above but canalso be made from a vascular graft preformed from a porous matrixmaterial such as ePTFE. One such graft is manufactured by W. L. Gore andmarketed as a Gore-Tex graft. The ePTFE graft may be sheathed over thepreviously described inner and intermediate components and heat fusedinto a similar composite graft described in this document. Similarly,the inner members may be a Gore-Tex graft, the intermediate component, aheat fusable thermoplastic, and the outer component, a Dacron knit.

Regardless of the configuration of the inner, intermediate and outercomponents of the graft, i.e. be it spun, salt eluted, phase inverted,wound with an outer PET mesh, or in which an ePTFE configuration isutilized, the resultant composite graft 21 as formed may be implanted invascular locations and retained in place through conventional methods,such as suturing. The preferred use of PET, knitted in a preferredtricot or double tricot pattern, from which to make the outer component61 of the graft 21 provides a graft having a greater thickness thangrafts without such a load bearing component. The outer component 61facilitates the greater retention of the sutures within the graft.

It will be understood that the embodiments of the present invention asdescribed are illustrative of some of the applications of the principlesof the present invention. Modifications may be made by those skilled inthe art without departure from the spirit and scope of the invention.

We claim:
 1. A mesh composite graft prepared by a process comprising thesteps of:(a) winding strands of biocompatible synthetic material over amandrel to form a cylindrically-shaped inner component having a lumentherethrough; (b) winding strands of biocompatible synthetic materialover an outer surface of said inner component to form an intermediatecomponent; (c) positioning an outer component comprising a preformedmesh of durable material over an outer surface of said intermediatecomponent; (d) said biocompatible synthetic material from which saidintermediate component is made having a melting temperature less thanthe durable material from which said outer component is made and lessthan the biocompatible synthetic material from which said innercomponent is made; (e) heating said components to a temperature greaterthan the temperature at which said biocompatible synthetic material fromwhich said intermediate component is formed melts but less than thetemperature at which said durable material from which said outercomponent is made melts and less than the temperature at which saidbiocompatible synthetic material from which said inner component is mademelts whereby said components are bound to each other; (f) cooling saidcomponents whereby said components are bound to each other by saidmelted intermediate component to form a totally porous compliant meshcomposite graft having a strengthened outer component.
 2. The meshcomposite graft prepared by the process according to claim 1, includingthe step of pressing said outer component into said intermediatecomponent during the heating step.
 3. The mesh composite graft preparedby the process according to claim 1, including the step of drying theintermediate component as wound over said inner component prior to theheating of said components.
 4. A method for forming a mesh compositegraft, which method comprises:winding strands of biocompatible syntheticmaterial to form a cylindrically shaped inner component having a lumentherethrough; winding strands of biocompatible synthetic material overan outer surface of said inner component to form an intermediatecomponent, positioning a preformed mesh made from strands of durablematerial over an outer surface of said intermediate component to form anouter component; said intermediate component material having a meltingtemperature less than the temperature at which the strands from whichthe outer component are formed melt and at which said biocompatiblesynthetic material from which said inner component is formed melts;binding said components together by heating said components to atemperature greater than the temperature at which said strands of saidintermediate component melt but less than the temperature at which saidstrands from which said outer component and said inner component areformed melt; and cooling said components as bound together to provide acompliant, totally porous mesh composite graft of said strands.
 5. Themethod according to claim 4, wherein said winding of said strands fromwhich said inner component is formed is carried out without interweavingsaid inner component strands.
 6. The method according to claim 4,wherein said winding of said strands from which said intermediatecomponent is formed is carried out without interweaving saidintermediate component strands.
 7. The method according to claim 4,wherein said preformed mesh of PET strands is formed by knitting saidstrands.
 8. The method according to claim 7, wherein said PET strandsare knitted in a tricot pattern.
 9. The method according to claim 7,wherein said PET strands are knitted in a double tricot pattern.
 10. Themethod according to claim 4, including the steps of securing said outercomponent to said intermediate component and said inner component bypressing said outer component into said intermediate component.
 11. Themethod according to claim 4, including the step of drying the strandsfrom which the intermediate component are formed immediately after saidwinding of said intermediate component.